1. Field of the Invention
The present invention relates to a test strip which allows a user to determine the concentration of an analyte in a liquid test sample. The test strip includes an absorbent membrane and a reagent that undergoes a change in color when exposed to the analyte, such as glucose or cholesterol, in a body fluid.
2. Description of Related Art
Test strips are commonly used for determining the concentration of analytes in liquid test samples. For example, known test strips can detect glucose, cholesterol, proteins, ketones, uric acid, phenylalanine, or enzymes in body fluids, such as blood or urine. The operation of such test strips has become so simple and reliable that the test strips can be used by patients to determine their own analyte concentrations.
Many test strips require a user to apply a drop of a body fluid to a reagent pad, or to a transport medium which conducts the fluid to the reagent pad, where an oxidizable dye or other indicator changes color to signal the presence of the analyte. It is sometimes difficult to bring exactly the right amount of body fluid to the test strip. If too little fluid is delivered, the test strip may not function properly. If too much fluid is applied to the test strip, excess fluid may drip from the test strip. Unnecessary contact between the user and the body fluid, which is typically blood, urine, or saliva, is preferably avoided.
The invention is directed to a color-indicating test strip which operates with a relatively small volume of body fluid but is capable of absorbing and retaining several times the required volume of body fluid, thereby minimizing any tendency for the body fluid to drip from the test strip. The test strip includes a membrane containing a color-changing reagent in contact with a porous sheet that has a pillow portion adapted to absorb excess body fluid from the membrane. The membrane extends across and protrudes into a window in the test strip. The window serves both to make visible any color change and to allow oxygen access to the membrane. The pillow portion is located in a protective channel defined by a relatively rigid container. The test strip is small, inexpensive, and suitable for mass production by heat-sealable packaging methods.
In one aspect, the invention is a test strip that comprises:
a cover sheet, having an elongated window cut through it;
a lamellar membrane adjacent to the cover sheet, extending across and into the window and containing a reagent tat reacts with the analyte to produce a color change;
a porous sheet in fluid communication with the membrane, having a pillow portion and a compressed portion, the pillow portion being substantially aligned with the window; and
a backing sheet, adjoining the porous sheet and having a sample port cut through it;
whereby fluid introduced into the sample port can flow to the membrane, and analyte in the fluid can react with the reagent to produce a color change visible through the window.
In another aspect, the invention is a test strip comprising a backing sheet. An opening through the backing sheet leads to an absorbent sheet of a relatively compressible nonwoven material, so that a liquid that passes through the opening is absorbed in the absorbent sheet. The absorbent sheet is sandwiched between the backing sheet and a relatively incompressible planar center plate. Because a central part of the center plate has been removed, as by punching, the center plate extends around an internal relief chamber. One of the faces of the center plate is fastened to the absorbent sheet. The other face of the center plate is attached to one side of a generally planar permeable membrane that has been treated with a reagent capable of changing color on contact with an analyte in a liquid solution. The side of the membrane is held by the face of the center plate, so that a portion of the membrane extends across and protrudes into the internal relief chamber of the center plate. A cover sheet defines a window that is generally parallel to and has a cross section which is approximately the same size as the internal relief chamber. The cover sheet is bound against the other side of the membrane with the portion of the membrane that is visible through the window bulging into the window.
The invention also provides a method of making a test strip. The cover sheet with the heat-sealable coating, the membrane, the porous layer, the adhesive, and the backing sheet are stacked as a heat-sealable package on a heatable shaping die. The die is generally flat, but has a groove adapted to accommodate the uncompressed porous layer in a region that is aligned with the window. The die is heated and forced against the package to compress portions of the membrane and porous layer and to bond the cover sheet and backing sheets in place.